In high-intensity discharge lamps, light is generated when an electric current is passed through a gaseous medium. The lamps have variable resistance characteristics that require operation in conjunction with a ballast to provide appropriate voltage and current limiting means. Control of the voltage, frequency, and current supply to the lamps is necessary for proper operation and determines the efficiency of the lamps. In particular, it determines the size and weight of the required ballast.
The appropriate voltage, frequency and current for efficient running of a lamp in its normal operating state is not appropriate for the lamp during its warm-up state. A high-intensity lamp typically takes several minutes to warm up from the time it is struck or turned on to its normal operating state. Initially, the lamp is an open circuit. Short pulses of current are sufficient to strike the lamp, provided they are of adequate voltage. Subsequent to striking, the lamp's resistance drops radically. The resistance then slowly rises during warm-up to its normal operating level. Hence, subsequent to striking and during warm-up the current of the lamp must be limited to prevent internal lamp damage.
Ballasts for high pressure sodium (HPS) lamps must be somewhat different from ballasts for other types of high intensity discharge lamps. First, the voltage required to strike a HPS lamp is much greater than that needed for other types of lamps. A short duration voltage pulse of over 2000 volts is needed for lower-wattage HPS lamps, and about 3000 volts is needed for 1000 watt HPS lamps. The need for a high voltage striking pulse typically requires a special starting circuit.
Second, it is a characteristic of a HPS lamp that its lamp voltage increases over the life of the lamp due to the slow increase in the stabilized temperature of its arc tube. Unless the HPS ballast maintains the lamp wattage, the HPS lamp output will vary beyond acceptable limits.